Circuit module assembly

ABSTRACT

A dual-board host or main printed wiring structure of a circuit module with increased component mounting area is made plug-compatible with a VME or VME64X backplane to which single-board host or main boards may be attached with connectors of differing dimensions and pin configurations, the center line of which are offset from each other. The connection sides of the two boards are respectively bonded to a heat sink frame structure having a thickness which corresponds to the connector offset on the backplane such that the total thickness of one of the printed wiring boards, the heat sink frame and the two bonding layers with respect to board to connector center line distances equals the connector offset at the backplane.

This application is a divisional application of co-pending applicationU.S. Ser. No. 09/076,625, filed on May 12, 1998.

DESCRIPTION

This application is related to co-pending U.S. patent applicationattorney docket no. FE-00332 filed concurrently herein and incorporatedby reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to electronic circuit moduleswhich are intended for mounting in a slot of a mechanical structurewhich provides electrical interconnection thereto and, moreparticularly, to electronic circuit board modules containing printedwiring boards.

2. Description of the Prior Art So-called printed circuit boards (PCB)or printed wiring boards (PWB) have been knownfor many years and may beformed by many techniques (e.g. screening, plating, etching, etc.).Printed wiring boards provide for a compact, structurally robust andeasily manufactured electronic circuit structure. Complexinterconnection patterns may be formed using multiple metal layersinterleaved with insulating layers in the board structure. Discreteelectronic components and integrated circuits (or sockets intended toreceive connections thereto) are affixed to the boards by solderingleads thereof to metallized pads on a surface of the board or insertingleads through holes in the printed wiring board.

As electronic systems, such as communication systems and dataprocessors, have become more complex, however, the use of multipleprinted circuit boards connected to each other has been convenient foraccommodating such systems in a compact enclosure or other physicalarrangement and limiting the size of printed wiring boards to limit thepotential for damage from vibration, impact and the like. Additionally,for ease of maintenance and repair, it has become common to fabricatecomplex systems in a modular form, often of standardized dimensions andto mount the modules in an interconnection structure which alsomechanically supports the printed wiring boards. Such structures may beof various forms referred to as card cages, backplanes, sub-racks andthe like. As these arrangements for providing interconnection andmechanical support for the modules have become more fully developed,they have also become largely standardized, placing limits on the sizeand shape which can be occupied by a module, including a printed wiringboard and components mounted on the printed wiring board.

At the same time, functionality and integration density of integratedcircuits has increased. Accordingly, the functionality and parts countof such modules has also increased both to exploit commerciallyavailable hardware dimensions and to avoid increasing complexity andrelative weight of the support and interconnection structure, as wouldoccur if modules were made smaller. Additionally, maintaining constantmodule size allows systems to be updated as improvements are made inindividual modules. Moreover, as circuit cycle and response speeds haveincreased with increased integration density, increased modulefunctionality allows connection length and signal propagation timewithin a module to be minimized.

Particularly in this last regard, it has become desirable to furtherincrease module performance while increasing module functionality byfurther increasing parts count in the modules to the extent possiblewithin the allowed volume of the module. Unfortunately, the limitingfactor is generally the area of the printed wiring board that may beaccommodated in a module of the mechanical support and interconnectionarrangement employed. Efforts to include additional printed wiringboards in a module have usually involved mechanical and/or electricalconnection of two boards but which are received in the support structurein the same manner as if the boards were separated and in differentmodules. U.S. Pat. No. 4,107,760 to Zimmer and U.S. Pat. No. 5,396,401to Nemoz are typical of such arrangements.

Further, heat dissipation requirements have increased with increasedswitching speed and functionality as discussed in U.S. Pat. No.5,483,420 to Schiavini which provides a heat sink plate in a singleboard module. In this regard, plural boards in a module have oftenrequired complex heat sink structures such as that disclosed in U.S.Pat. No. 4,916,575 to Van Asten.

At the present time, a prevalent standard for modular circuit packagingis commonly known as the "Versa Module Europa" (VME) which incorporatesa number of design standards including those known in the art as IEEEstandard 1101.1, IEEE standard 1101.2, VITA20, ANSI/VITA1, VITA1.1(VME64X) and IEEE standard P1386.

While these standards allow some degree of freedom in the mechanicaldesign and component layout within the module, all of the standardsinvolved in the collective VME standard are directed to modulesincluding only a single printed wiring board having components mountedon only a single side thereof. The placement of the board in modules inaccordance with the VME standard allows a component height ofapproximately 0.520 inches on one side of the single board whileallowing a very low component height/pin protrusion height ofapproximately only 0.075 inches on the opposite "connection" side of theboard. This very limited pin protrusion height precludes mounting ofmost common integrated circuit components on the connection side of theboard. (Modern surface mount technology (SMT) components generally havea height between 0.090 and 0.160 inches.) Therefore, the area availablefor component mounting is currently a major impediment to increasingmodule functionality and parts count. By the same token, since the VMEstandard contemplates only a single board per module and module mountingat 0.800 inch pitch, the volume available for employment of heat removalstructures is also quite limited.

It is also known, as discussed in U.S. Pat. No. 4,879,634, to augmentthe area of the single VME standard board with an additional board knownas a mezzanine board. (When a mezzanine board is present, the main boardof the VME module is referred to as the "host" board since the mezzanineboard essentially plugs into the host board. The term "host" is alsoused synonymously At with "main" board whether a mezzanine board isemployed in a module or not.) Input and output (I/O) connections to themezzanine board from the interconnection structure are made through thehost board. This configuration increases the necessary length ofconnection paths and also places an extra plug and socket or otherunsoldered connection arrangement in the signal path which may be asource of noise or signal path discontinuities, possibly intermittentwith vibration, acceleration or differential thermal expansion.

Further, while the mezzanine board has been used successfully in manymodule designs, it can be readily seen that it occupies volume thatcould otherwise be used for heat removal structures such as heat sinks.Moreover, the mezzanine board is commonly attached to the host board, inaccordance with the VME collective standard, such that the componentside of the mezzanine board generally faces the component side of thehost board and thus tends to aggravate the occurrence of hot spots onthe host board. The components on the mezzanine board may interfere withair circulation over the surface of any heat sink which may be providedwhile the proximity of components on the mezzanine board to the hostboard provide localized heat input thereto. Modification of such a heatsink may also be required to accommodate components on the mezzanineboard and in any case, the area and coverage of the heat sink must bereduced to accommodate connections to the host board.

In summary, a mezzanine board can be used to supplement the area of asingle printed wiring board host provided in accordance with the VMEstandard but is not an ideal solution since its use increases length ofsignal and power connections, requires connections which are notsoldered and interferes with heat sinking of the host board whileincreasing the likelihood that hot spots will develop on the host board.Further, due to the unsoldered connections, the difficulty ofmechanically supporting a mezzanine board, the requirement thatconnections to the mezzanine board be made through the host boardconnectors and the volume which is occupied by a mezzanine board, onlyone mezzanine board can be used with a host board; limiting the totalmodule area to less than twice the host board area. However, the use ofa mezzanine board has been the only practical structure to approach theproblem of supplementing printed wiring board area within the rules ofthe VME standard.

The principal reason that the mezzanine board has been the onlypractical approach to increasing module functionality and parts count isthe geometrical configuration of host board connectors in accordancewith the VME standard. Specifically, to obtain a sufficient number ofcontact pins, an entire edge of the VME printed wiring board ispopulated with connection pins grouped into three connectors. As the VMEstandard was developed, commercially available edge connectors wereemployed. Two of the connectors which are adjacent corners of the VMEprinted wiring board (designated the P1 and P2 connectors) includedthree or five rows of pins while the central connector (designated P0)included five rows of I/O pins with an additional two rows of groundpins.

As commercial connectors having these configurations were mounted on thecomponent side of the VME printed wiring board, the center line of thecentral P0 connector was thus offset from the center line of the P1 andP2 connectors (by 1.85 mm or 0.073 inches). This offset also, in time,became part of the VME standard and could not be met by connectorsattached to more than one printed wiring board of a module. Therefore,the VME standard could not be met for a second printed wiring boardunless connections were provided through a host board, as in themezzanine board configuration discussed above. No further augmentationof printed wiring board area with additional mezzanine boards ispossible for the reasons discussed above and therefore even thenon-ideal mezzanine board solution is limited to less than doubling theavailable area of the host board while presenting further problems whichmay compromise performance of the module or reliability of a module insevere environmental conditions of vibration, acceleration or thermalexcursions.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a circuitmodule structure which is plug-compatible with the VME standard whileallowing available printed wiring board area in a module to exceed thatof a single printed wiring board host.

It is another object of the invention to provide a circuit modulestructure which is plug compatible with the VME or other standardgeometry and having improved heat distribution and removalcharacteristics which avoid hot spots on the host board.

It is a further object of the invention to provide a dual board hoststructure with direct connections to two printed wiring boards in amodule which is plug-compatible with the VME standard.

In order to accomplish these and other objects of the invention, aprinted wiring board and heat sink structure comprising first and secondprinted wiring boards, each having a component side and a connectionside. A heat sink frame of thermally conductive material having a firstside and a second side is bonded to the first and second printed wiringboards by a first bonding layer and a second bonding layer,respectively. The bonding layers are between the connection side of thefirst and second printed wiring boards and the sides of the said heatsink. An offset between mounting surfaces on said first and said secondprinted wiring boards are positioned such that when connectors aremounted to appropriate printed wiring board surfaces, the center linesof the connectors correspond to an offset between center lines ofconnectors on a single slot location on a backplane to which saidconnectors may be connected is also provided.

The connector offset on the backplane equals a total thickness of saidsecond printed wiring board, a thickness of the heat sink frame andthicknesses of the first and said second bonding layers with respect tothe first and the second printed wiring board to respective connectorcenter line distances.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIGS. 1A and 1B are isometric views of the A and B sides of a dual board(host) module in accordance with the invention, respectively,

FIGS. 2A, 2B and 2C are plan (side A), side and end views of a dualboard (host) module in accordance with the invention, respectively,

FIGS. 3A and 3B are a plan view of side B and a front/top side view,respectively, of a dual board (host) module in accordance with theinvention,

FIG. 4 is an exploded isometric of a dual board module corresponding tothe view of FIG. 1A,

FIG. 5 illustrates mounting of two modules on a support andinterconnection structure in accordance with the view of FIG. 2C, and

FIG. 6 is an exploded isometric view of a preferred form of a mezzanineboard mounting on the dual board host module of FIG. 1A.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 1A and 1B,there is shown, in isometric view, respective sides of a dual boardstructure which is plug-compatible with the VME standard in accordancewith the invention. The respective sides of the dual-board structure andthe boards 10, 12 constituting respective sides of the structure arereferred to as the "A" and "B" sides which are respectively visible inFIGS. 1A and 1B. These printed wiring boards 10, 12 have theirconnection (frame interface) sides bonded (e.g. with a layer ofinsulating adhesive or an adhesive coated insulating sheet 10a, 12a) toa preferably solid and continuous heat sink plate or frame 20 which hasends 22 including ribs 22' which are shaped to fit into slots in a cardcage having a backplane 100 (with connectors 110 to receive connectors14, as shown in FIG. 5), card cage, sub-rack (either being schematicallyrepresented at 120 of FIG. 5) or the like preferably in accordance withthe collective VME standard discussed above.

FIGS. 1A and 1B further show a wedge lock expander 40 positioned on anedge of the boards 10, 12. Also shown in FIG. 1B are mezzanine boardconnectors 60 which connect the board 12 to a mezzanine card. Themezzanine card connectors 60 are preferably VME industry standardconnectors 60, and are used in one embodiment of the present invention.

Ends 22 are also preferably shaped and include ribs 22' located toprovide approximately 0.160 inch component height above the A side boardand approximately 0.300 inch component height above the B side boardwithin a standard thickness (e.g. 0.70 inches) leaving a standardclearance (e.g. 0.10 inches) between modules when ribs 22' engage slotsin a card cage or sub-rack or the like. However, it should be understoodthat the respective component heights provided in this manner can bedivided in any desired manner but are chosen for convenience of plugalignment, as will be discussed in greater detail below, and since 0.160inches will accommodate virtually all commercially available flat packsand 0.300 inches is adequate to accommodate most discrete componentscommonly used in modular circuit constructions. Larger components can beaccommodated as necessary within the 0.800 card pitch of the VMEstandard or any other standard or spacing by cutting apertures in one orboth boards 10, 12 and, if both, heat sink 20.

Other preferred mechanical features of the invention include extractorlevers 30 cooperating with "wedgelock" expanders to firmly retain themodule in a card cage or the like and backplane 100 while providing forease of removal therefrom without damage to the module or itsconnectors. A test connector 50 may also be provided on either or bothof boards 10, 12 with the other board being notched, as needed, as shownat 52. Crossover connections 18 between boards can also be included andcan be permanently made, such as by soldering since the PWBs 10, 12 andheat sink frame 20 are bonded together as a unit. None of these featuresare at all critical to the practice of the invention and can be omitted,included or modified as desired within the principles of the invention.

As alluded to above, the VME standard was developed from the use ofcommercially available connectors mounted on a common side of a singleboard, resulting in a 1.85 mm or 0.073 inch offset in the center line ofthe central P0 connector relative to the center lines of the P1 and P2connectors (this 1.85 mm is VME specified). It was also noted above thatdirect connections from the connectors to each of the respective PWBs inaccordance with the invention would avoid the need for additionalunsoldered connections required for other boards and for reducingconnection length.

In accordance with the invention, the thickness of the heat sink frame20, the thickness of board B, 12, and the thickness of the insulatingbonds of the boards 10, 12 to the heat sink frame 20 are adjusted, inaccordance with the invention, so that when the P1 and P2 connectors 14are attached to the connection/wiring side of board A, 10, with leadconnections 16 and the P0 connector 24 attached to the component side ofboard B, the center lines of the connectors are respectively offset tomatch a desired standard, such as the VME standard, and thus achieveplug-compatibility with the backplane in accordance with the samestandard while providing direct connections to both boards 10, 12, asshown in FIGS. 2B, 2C and 3B. Reference numeral 24a is a representationof an offset of the connectors 24 and 14. For the preferred VMEstandard, a heat sink frame thickness of 0.076 inches in the PWB areas,a bond thickness of 0.004 inches and a board B thickness of 0.061 inchesis preferred while allowing use of commercially available connectors.

That is, the total thickness of the heat sink frame, two bonds to theboards 10, 12, the thickness of board B, 12, and the distance from boardB component side surface to connector center line of connector 24 (sinceconnector 24 is placed on the component side of board B, opposite theheat sink frame) less the distance from board A connection side surfaceto connector center line of connectors 14 (since they are placed on theconnection side of board A, 10, as opposed to the component side inaccordance with the VME standard) equals the desired offset of thecenter lines of the connectors (0.073 inches (1.85 mm) in the VMEstandard) that exists between the connectors on backplane 100. Putanother way, the total thickness of board B, 12, the thickness of theheat sink frame and the two bonding layers with respect to the boards toconnector center line distances equals the connector offset on backplane100 and thus matches the offset which would occur if the connectors 14and 24 were mounted on the same surface of a single printed wiring board(which is the industry standard). In other words, the first and thesecond printed wiring boards are positioned such that when connectorsare mounted to appropriate surfaces of the first and the second printedwiring boards, respectively, center lines of the connectors equal anoffset on a backplane to which the connectors are connected thereon.

The above thicknesses and their relationships are illustrated in FIG. 5.The heat sink frame thickness, if fabricated of a material havingrelatively good thermal conductivity such as copper or aluminum, hasbeen found adequate to provide enhanced thermal performance andavoidance of hot spots by being sufficiently conductive of heat tomaintain substantially uniform temperature and uniform heat spreadingacross both boards as well as to conduct heat to the card cage throughedges 22.

It should be noted that while standard commercially available connectorsare used for the P1 and P2 connectors 14, their pin numbers must berotated 180° (at the board design level) since they are mounted on theconnection side of PWB A, 10, rather than the component side as in theVME standard. A similar rotated pin labeling scheme. applies to the P0connector, being that it is mounted to a wholly different board.Mechanical keying of the connectors, if used, must also be similarlyrotated. However, it should be understood that pin numbering, keying andthe side of the board on which a given connector is intended to bemounted is largely a matter of convention or labeling and customconnectors could also be used.

For example, connector 24 overlaps the edge of board 12 as bestillustrated in FIG. 4 is mounted on the component side of the board 12as is intended by the manufacturer. However, connectors 14 are intendedto be mounted on the component side of the same board but are mounted onthe connection side of board 10 in accordance with the invention.However, a commercially available or custom connector could be usedwhich was intended for connection to the connection/wiring side of thePWB which would, for that reason, already include the rotation of pinnumbers and keying.

It should also be noted that boards 10 and 12 are sized and notched, asillustrated at 26, 28, respectively, of FIGS. 2A, 3A and, particularly,the exploded view of FIG. 4, to provide clearance for the mounting ofconnectors 14 and 24. (Depending on connection requirements, however,connector 24 may not be required on all modules.) These connectors maybe mechanically attached to the board and heat sink laminated "sandwich"structure by any desired method which provides adequate strength. Theuse of small bolts, as illustrated, is preferred to transfer forces(e.g. during insertion and removal from backplane 100 and connector 110)to the heat sink frame 20.

FIG. 4 further shows ends 22 of heat sink frame 20 are offset from thesurfaces to which boards 10, 12 are bonded to provide the componentclearances discussed above in accordance with protrusions which engage acard cage or the like. FIG. 4 further shows top ends 62 of the heat sinkframe 20.

FIG. 6 shows an exploded isometric view of a preferred form of amezzanine board mounting on the dual board host module of FIG. 1A. Morespecifically, FIG. 6 shows a mezzanine board 80 mounted to the board 12via the heat sink frame 20. The mounting of the mezzanine board 80 onthe heat sink frame 20 maximizes the volume usage and availability ofthe component side of the host board structure such that components canbe mounted within the clearance created between the facing surfaces ofthe mezzanine board 80 and the board 12. This mounting assembly furtherprovides additional protection for components mounted thereon when themodule is removed from the card cage and allows the plug and socketconnection to be mounted on the component side of the mezzanine boardfor convenience of manufacture. In the embodiments of the presentinvention, the mezzanine board 80 may also be mounted to the connectors60.

In view of the foregoing, it is seen that the invention including a dualboard laminated structure provides plug-compatibility with existingmodule dimension standards such as the VME standard while providing astructurally robust structure of improved electrical reliability andperformance and much increased thermal performance and freedom from hotspots while effectively doubling the board area for mounting ofcomponents on the host circuit card.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is as follows:
 1. A circuit module comprising:afirst printed wiring board having a component side and a connectionside, said first printed wiring board further having opposing side edgesand a connection edge between said opposing side edges; heat sink ribseach having a first surface and a second surface, said first surface ofeach of said heat sink ribs being mounted along said opposing side edgesof said first printed wiring board outside a component mounting area onsaid component side of said first printed wiring board; a second boardhaving opposing side edges, said opposing side edges of said secondboard being mounted along said second surface of each of said heat sinkribs such that said opposing side edges of said first printed wiringboard and said opposing side edges of said second board are mounted tosaid opposing heat sink ribs at said first and second surfaces thereof,and such that a clearance is provided between said second board and saidcomponent side of said first printed wiring board so that components canbe mounted on said component side therebetween.
 2. The circuit module ofclaim 1, wherein said second board is a mezzanine board.
 3. The circuitmodule of claim 2, wherein said mezzanine board is a triple sizemezzanine board.